1. Field of the Invention
The present invention relates to a single substrate, microwave radar transceiver including flip-chip mounted monolithic microwave integrated circuit (MMIC) chips for applications such as near obstacle detection systems (NODS) for automotive vehicles.
2. Description of the Related Art
Conventional MMICs are fabricated on gallium arsenide (GaAs) substrates using the microstrip line as the principal microwave signal transmission medium. A general treatise on MMIC technology and microwave transmission line configurations is found in "Millimeter-Wave Integrated Circuits", by Kai Chang, TRW- Electronics & Defense Sector/Quest, Winter 1983/84, pp. 43-59. Radar transceivers incorporating MMIC integrated circuits are desirable in numerous applications, including NODS, true ground speed sensors, obstacle avoidance systems (adaptive cruise control), and active phased array radars which utilize a large number of transceivers in a single operating unit.
The thickness of the GaAs substrate is typically limited to 100 microns at X-band and higher frequencies based on dispersion characteristics, mode conversion, thermal and circuit density considerations. These MMIC chips are too fragile for automated chip handling using modern robotic manufacturing techniques such as pick and place and die bonding. In addition, wire bonding interconnects with 25 micron diameter gold wires are generally used, which are labor intensive and lead to reliability problems. Microstrip based MMICs are not compatible with low cost flip-chip assembly technology because the ground plane is on the opposite side of the substrate from the microwave frequency electronic circuit elements.
Microstrip is the most widely used transmission line in both hybrid and monolithic microwave integrated circuits. As illustrated in FIG. 1, an electrically conductive stripline 10 is formed on one surface of an electrically insulative or dielectric substrate 12, whereas an electrically conductive electrode or ground plane 14 is formed on the opposite surface. The characteristic impedance of the microstrip transmission line is determined by the width of the stripline 10, and the thickness and dielectric constant of the substrate 12. The thickness of the substrate 12 is usually kept to a small fraction of a wavelength of the highest signal frequency propagating in the substrate 12 to avoid excess frequency dispersion or undesirable higher order mode (other than the fundamental transverse electrical and magnetic (TEM) mode) excitation at the signal frequency, or harmonics of the signal frequency. Typical GaAs based MMICs operating at X-band and higher frequencies are fabricated on 100 micron thick substrates. Access to the ground plane 14 is provided by means of a metallized vertical interconnect (via) formed in a hole extending through the substrate 12.
As illustrated in FIG. 2, a conventional MMIC hybrid microcircuit arrangement includes individual MMIC chips 16, 18 and 20 which are mounted on a common metal substrate or carrier 22 maintained at ground potential. Interconnects between the chips 16, 18 and 20 are provided by gold wires or ribbons 24 which are typically 25 microns in diameter. The interconnects 24 are often the major source of reliability problems when the chips 16, 18 and 20 are assembled into a common module with other integrated circuits. The thin (100 micron thick) chips 16, 18 and 20 are too fragile for fabrication processes using automated/robotic pick and place techniques. In addition, the microstrip based circuitry as illustrated in FIG. 1, with the ground plane 14 on the side opposite the striplines 10, is not compatible with flip-chip assembly techniques using a low cost reflow solder process.